> Hard Mode means that you split your program into std binary and #![no_std] no-alloc library. Only the small binary is allowed to directly ask OS for resources

> Hard Mode means that you split your program into std binary and #![no_std] no-alloc library. Only the small binary is allowed to directly ask OS for resources. For the library, all resources must be injected. In particular, to do memory allocation, the library receives a slice of bytes of a fixed size, and should use that for all storage. I did exactly this in my QR Code generator library, C port and second Rust port: https://github.com/nayuki/QR-Code-generator/blob/master/c/qrcodegen.h https://github.com/nayuki/QR-Code-generator/blob/master/c/qr... , https://github.com/nayuki/QR-Code-generator/blob/master/rust-no-heap/src/lib.rs https://github.com/nayuki/QR-Code-generator/blob/master/rust... Writing the Rust code felt way safer because the language took care of enforcing rules about borrowing parts of the buffer.